The invention relates to a circuit arrangement for supplying a constant voltage to electric loads, where at least one load is connected via a switching regulator to a power feed loop fed with a constant current. The switching regulator comprises a switching element controllable by means of a control circuit as a function of a voltage which is to be maintained.
Such a circuit arrangement is known from, for example, German patent DE-AS No. 1,947,901.
The known circuit arrangement is intended in particular for a constant load. In normal operation the output voltage is maintained constant by means of a series regulator. The switching regulator is active only below a given load current, or in particular, in the case of a load removal.
For electrical systems with interlinked loads it is often advantageous to supply the loads with constant current over a common line. Such power supply arrangements are frequently used in long-distance traffic equipment. The individual line amplifiers or regenerators are connected in series and are powered from one or two feeding terminal stations. Since in normal operation they behave as an almost constant ohmic load, a certain, largely constant voltage will occur in the system as a whole.
Most electronic devices can best be operated with constant voltage. However, they then have a correlated current consumption, which is load-dependent. To permit current feed, the feed device must be rated so that the desired operating voltage occurs at the current then given. If it is rated for a smaller current the excess current is removed via a zener diode connected in parallel.
The rating of the loads presents a problem in particular when they are subjected to large variations as when it is necessary to operate equipment having different power requirements, and/or when several equipments are to be connected in parallel in different arrangements. In such cases it is desirable to interject in front of the actual loads converters which produce a constant voltage from a constant current and which then, in accordance with current consumption operate with different input voltages.
An arrangement which in a lower range of output power operates with a switching regulator is described in German patent document DE-AS No. 1,947,901. In the known arrangement it is assumed that at large load variations neither the swtiching regulator principle nor the series pass regulator principle by itself is usable without considerable disadvantages. The arrangement, therefore, is such that the regulation process adjusts itself to the more favorable control principle or respectively to a combination of the two regulation principles depending on the operational case at hand. With a view to desired dynamic properties, the operating point in rated duty lies in the control range of the series pass regulator. For relatively large control ranges, however, this may lead to an unacceptably high power dissipation.
From Siemens-Zeitschrift 48 (1974), Supplement "Nachrichten-Ubertragungstechnik", page 130, there is known further a power supply system in which the transfer factor of transformers of an auto-converter can be adjusted so that the voltage drop at the load resistance has a given value. Here the auto-converter is adjustable to a current transfer corresponding to the particular use. For regulating voltage a voltage regulator of the series pass type is used.
The current transfer can be adjusted to the optimum value automatically in steps with the aid of a switching device. However, this involves additional expense.
The generally accepted converter principles are based on the assumption that converters are operated with at least approximately constant voltage. Generally, they have several disadvantages when they are operated with a load-independent input current and are required to deliver a constant voltage for a varying load.
Upon being switched on, the converters begin to operate at a voltage which must be lower than the lowest input voltage to be expected in the steady state. Without additional measures, the usual converters try to produce the nominal voltages directly at the output, the internal tank and filter capacitors being charged additionally. Consequently, the current drain is especially high during the initial turn on. But if the developed current is limited, as is the case with a constant current source, the voltage will collapse at the output. In such cases, the converters cannot be switched on.
Also at greatly fluctuating loads it may occur that the voltage collapses at the input because of the current limitation. This can be avoided only through additional measures, e.g. by over designing the converters as referred to the nominal power.
The two cases described can be mastered according to German patent document DE-AS No. 1,947,901 by a control circuit which, however, is relatively expensive and moreover has the property that rapid load fluctuations are intercepted by a series pass regulator and the mode of operation is changed relatively slowly in such a way that the converter is operated with the then correspondingly adapted voltage at the input.